Hi, thanks for your question. My name isXXXXX can answer this for you.
Did the contract of sale contain any right to cancel?
No It simply says that it is up to me to check the dimensions before I sign. I thought it would be ok but it was not
Okay. Then there is no legal right to cancel. You only get a statutory right to cancel when you enter into a contract via some "distance communication" such as by email or telephone. If you enter an agreement face-to-face, then there is no legal right to cancel unless it's agreed in the contract itself. I'm sorry that's not what you wanted to hear.
Do I not have a cooling off period?
If they had told you that you had the right to cancel at the show, that would have been different.